How many of us are guilty of the confusion in this – is it a million-dollar smile or million dollars smile? not many, because this is something that has been taught since childhood, however, what we do not know is the reason why it is a million-dollar smile and not a million dollars smile, another example of similar words is – 5-star hotel and 5-star chocolate.

Compound nouns are words for people, animals, places, things, or ideas, made up of two or more words. Most compound nouns are made with nouns that have been modified by adjectives or other nouns.

In many compound nouns, the first word describes or modifies the second word, giving an insight into what kind of thing an item is, or providing us with clues about the item’s purpose. The second word usually identifies the item.

The contractor decided to construct a 40 house/houses building.

This statement might cause a little bit of confusion. The compound noun here should be: forty house building.

Million-dollar smile – million dollars is the adjective, white the smile is the noun.
5 star hotel – 5 star is the adjective, while the hotel is the noun.
40 house building – 40 house is an adjective, while the house is the noun.

Thus, the basic rule is adjectives can never be plural, and hence the adjective part of the compound noun is always singular.